Over 170 Separex™ membrane systems have been installed in the world for gas separation applications such as for the removal of acid gases from natural gas, in enhanced oil recovery, and hydrogen purification. Two new Separex™ membranes (Flux+ and Select) have been commercialized recently by Honeywell UOP, Des Plaines, Ill. for carbon dioxide removal from natural gas. These Separex™ spiral wound membrane systems currently hold the membrane market leadership for natural gas upgrading. These membranes, however, do not have outstanding performance for olefin/paraffin separations. Development of new stable and very high selectivity membranes is critical for the future success of membranes for olefin/paraffin separation applications such as propylene/propane and ethylene/ethane separations.
Light olefins, such as propylene and ethylene, are produced as co-products from a variety of feedstocks in many different processes in the chemical, petrochemical, and petroleum refining industries. Various petrochemical streams contain olefins and other saturated hydrocarbons. Typically, these streams are from stream cracking units (ethylene production), catalytic cracking units (motor gasoline production), or the dehydrogenation of paraffins.
Currently, the separation of olefin and paraffin components is performed by cryogenic distillation, which is expensive and energy intensive due to the low relative volatilities of the components. Large capital expenses and energy costs have created incentives for extensive research in this area of separations, and low energy-intensive membrane separations have been considered as an attractive alternative.
In principle, membrane-based technologies have the advantage of both low capital cost and high-energy efficiency compared to conventional separation methods for olefin/paraffin separations, such as propylene/propane and ethylene/ethane separations. Four main types of membranes have been reported for olefin/paraffin separations. These are facilitated transport membranes, polymer membranes, mixed matrix membranes, and inorganic membranes. Facilitated transport membranes, or ion exchange membranes, which sometimes use silver ions as a complexing agent, have very high olefin/paraffin separation selectivity. However, poor chemical stability, due to carrier poisoning or loss, high cost, and low flux, currently limit practical applications of facilitated transport membranes.
Separation of olefins from paraffins via conventional polymer membranes has not been commercially successful due to inadequate selectivities and permeabilities of the polymer membrane materials, as well as due to plasticization issues. Polymers that are more permeable are generally less selective than are less permeable polymers. A general trade-off has existed between permeability and selectivity (the so-called “polymer upper bound limit”) for all kinds of separations, including olefin/paraffin separations. In recent years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success in terms of improving the membrane selectivity.
More efforts have been undertaken to develop metal ion incorporated, high olefin/paraffin selectivity facilitated transport membranes. The high selectivity for olefin/paraffin separations is achieved by the incorporation of metal ions such as silver (I) or copper (I) cations into the solid nonporous polymer matrix layer on top of the highly porous membrane support layer (so-called “fixed site carrier facilitated transport membrane”) or directly into the pores of the highly porous support membrane (so-called “supported liquid facilitated transport membrane”) that results in the formation of a reversible metal cation complex with the pi bond of olefins, whereas no interaction occurs between the metal cations and the paraffins. Addition of water, plasticizer, or humidification of the olefin/paraffin feed streams to either the fixed site carrier facilitated transport membranes or the supported liquid facilitated transport membranes is usually required to obtain reasonable olefin permeances and high olefin/paraffin selectivities. The performance of fixed site carrier facilitated transport membranes is much more stable than that of the supported liquid facilitated transport membranes and the fixed site carrier facilitated transport membranes are less sensitive to the loss of metal cation carriers than the supported liquid facilitated transport membranes.
Pinnau et al. disclosed a solid polymer electrolyte fixed site carrier facilitated transport membrane comprising silver tetrafluoroborate incorporated poly(ethylene oxide), see U.S. Pat. No. 5,670,051. Herrera et al. disclosed a process for the separation of olefin/paraffin mixtures using a silver cation-chelated chitosan fixed site carrier facilitated transport membrane, see U.S. Pat. No. 7,361,800. Herrera et al. also disclosed the coating of a layer of chitosan on the surface of a microporous support membrane, where the support membrane is made from polyesters, polyamides, polyimides, polyvinylidene fluoride, polyacrylonitrile, polysulfones or polycarbonates.
Feiring et al. disclosed a new facilitated transport membrane comprising silver (I) cation exchanged fluorinated copolymer synthesized from a perfluorinated cyclic or cyclizable monomer and a strong acid highly fluorinated vinylether compound, see US 2015/0025293.
The composite facilitated transport membranes disclosed in the literature use an ultrafiltration or microfiltration membrane as the support membrane. However, the use of a relatively hydrophilic, nanoporous polymeric membrane such as a polyethersulfone membrane as the support membrane for the preparation of fixed site carrier facilitated transport membranes for olefin/paraffin separations has not been reported in the literature. In particular, the use of a relatively hydrophilic, very small pore, nanoporous support membranes with an average pore diameter of less than 10 nm on the membrane skin layer surface for the preparation of fixed site carrier facilitated transport membranes has not been disclosed in the literature.
Development of new stable, high permeance, and high selectivity facilitated transport membranes is still required for the use of membranes for olefin/paraffin separations such as propylene/propane and ethylene/ethane separations.